Captcha code no longer working - "Invalid Captcha Code" error

I had wpDiscuz working perfectly with the captcha code on my blog until very recently. It appears to have been broken either by the latest Wordpress Update (4.7.2) or perhaps by an update to one of the other plugins I'm using. My readers reported the problem to me, and I hadn't changed any of the plugin's settings to cause it to break.

Currently have wpDiscuz 4.0.4 installed, with no add-ons. Whenever you submit a comment, the Captcha code resets, and the error message "Invalid Captcha Code" appears. I've tried both the filesystem and Wordpress session captcha types, and neither one works. I don't see any obvious JS errors or other clues about what is happening. I have not changed any of my Wordpress configuration files since the captcha was working, except to update the Wordpress engine itself and my various plugins to their latest versions.

For now I've managed to disable the captcha to get comments working again, but I don't want to leave it disabled due to possible spam. Please help - I like wpDiscuz but I can't have it breaking on me so easily. Comment systems need to just *work*.

Suggestions from other users who have had this problem also welcome.

Thanks!